McNulty grew up in Phoenix, Arizona, racing mountain bikes before he took up road racing as a teenager. He won the junior time trial at the 2016 World Championships in Doha, beating Mikkel Bjerg by 35 seconds. WorldTour offers followed and he turned them down, signing instead for Rally Cycling, an American Continental team, where he added the under-23 national time trial title in 2017.
He stayed at Rally for three seasons. His first professional win came at the 2019 Giro di Sicilia, where he took the stage into Ragusa and then held his lead on Mount Etna, finishing 42 seconds clear of Guillaume Martin overall. UAE Team Emirates signed him for 2020 and he rode his first grand tour at that year's Giro d'Italia, finishing 15th.
At the Tokyo Olympics in 2021 he attacked with 25 kilometres left and only Richard Carapaz went with him. Carapaz dropped him inside the final six kilometres and the chase group swept him up, leaving him sixth. At the Tour de France a year later he set the pace on the climb to Peyragudes until only Tadej Pogačar and Jonas Vingegaard were left, then held on for third on the stage.
He rode clear at Paris-Nice in 2022 to take stage five by almost two minutes, and in 2023 he beat Ben Healy and Marco Frigo out of a three-man break on stage 15 of the Giro d'Italia. He won the American time trial title in 2023 and 2024, and was fourth at the world championship time trial in Stirling in 2023.
In 2024 he won the Volta a la Comunitat Valenciana in February, took GP Miguel Induráin as the first American to win it, finished third at Paris-Nice behind Matteo Jorgenson and Remco Evenepoel, and won the opening time trial of the Vuelta a España in Lisbon. Those wins, and most of his others, came from time trials or from long efforts held at one pace rather than from sprints or late attacks.
He won the Tour de Pologne in 2025 on the last day, starting the closing time trial in Wieliczka seven seconds down on the lead and finishing 29 seconds clear of Antonio Tiberi.







